Single-channel ligand exchange for W401F-CFTR. (A and C) A single W401F-CFTR was activated by ATP, and the ligand was switched to a 1-s PPi (A) or AMP-PNP (C) pulse in the open state. The channel was locked open after the short application of PPi or AMP-PNP. The current trace was expanded in the red box to better discern events during ligand switch. (B and D) The same 1-s PPi (B) or AMP-PNP (D) pulse was applied when the channel resided in the closed state; no locked-open event was seen. Channels can be reactivated by ATP after long washout.